: This is a revised application from Dr. William Green to study anti-AKR/Gross MuLV CTL: Mechanisms of nonresponsiveness. The development of immunological strategies that will ensure the generation of sufficient immunity towards transformed cells is one of the major unresolved issues in the field of cancer research. In this proposal we intend to extend our ongoing studies which utilize weakly immunogenic, AKR/Gross murine leukemia virus (MULV) induced tumor cells as a model system for examining cell-mediated anti-tumor immunity. In particular, the generation of cytolytic T lymphocytes (CTL), an effector cell type generally considered to be especially beneficial against virus-infected cells and tumor cells, will be emphasized. This proposal will focus on the inability of two congenic strains of mice, AKR.H-2b and AKR.H-2b:Fv-lb, to mount AKR/Gross MULV specific CTL responses. Our focus on these congenic mouse strains is part of our general approach towards dissecting the importance of antiviral CTL in this system by sequentially moving from our earlier studies with low leukemic/high CTL responder C57BL/6 (B6, H-2b) mice to high leukemic/low responder AKR (H-2k) mice. For B6 prototypic anti-AKR/Gross MULV CTL, we have shown that the type-specific viral TM 134-141 peptide, KSPWFTTL, represents the immunodominant epitope. In contrast, AKR.H-2b mice, despite the presence of a responder H-2b haplotype, fail to mount such antiviral CTL responses. Young AKR.H-2b:Fvlb mice, whose additional differential locus (Fv-1b) retards the expression and spread of endogenous N-ecotropic proviruses such as EMV- II, are able to generate anti-AKR/Gross virus CTL. As mice of this latter double congenic strain age, however, viral antigen expression increases, and beginning at about 9 weeks of age they become specifically unable to generate such antiviral CTL. Our findings indicate that clonal deletion of the precursors of antiviral CTL is not operative in these strains. Furthermore, moderately-aged AKR.H-2b:Fv-lb mice contain antigen specific CD8+ inhibitory T cells in adoptive transfer experiments. CD8+ T cell and other lymphoid inhibitory cells have also been demonstrated in vitro and in vivo in the AKR.H-2b strain. Similarly, CTL responder B6 or B6.Fv-ln mice infected exogenously with certain MuLV become specifically unable to generate AKR/Gross MuLV-specific CTL. This proposal seeks to characterize the mechanisms of action of the inhibitory cells which are responsible for CTL peripheral nonresponsiveness, specifically focussing on the common mechanism in these systems which we have recently shown is FasL/Fas mediated activation- induced cell death (AICD) induced by viral antigen and FasL expressing normal lymphoid cells. The experimental approach will be to use the previously defined and easily manipulated in vitro system in which the secondary restimulation of primed responder B6 antiviral pCTL/CTL is inhibited by normal AKR.H-2b spleen cells, as well as to selectively employ the in vivo systems. It is hoped that these studies will add to our understanding of the factors controlling responsiveness to weakly-immunogenic tumors and how retroviruses interact with and modulate the immune system by establishing peripheral tolerance mechanisms.